Step 1: Note the real Cup A scores.
Cup A scores are India 324, Australia 309, South Africa 315 and Sri Lanka 280.
Step 2: Apply the given condition.
The question replaces India's Cup A score with South Africa's Cup C score, which is 255.
So the working Cup A scores become Australia 309, South Africa 315, Sri Lanka 280 and India 255.
Step 3: Rank the four teams by score, highest first.
South Africa 315 takes rank 1.
Australia 309 takes rank 2.
Sri Lanka 280 takes rank 3.
India 255 takes rank 4.
Final Answer:
With India's score replaced by 255, India finishes last in Cup A, at rank 4. \[ \boxed{4} \]